In the manufacture of paper products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives. Examples of such chemical additives include softeners, humectants, debonders, wet strength agents, dry strength agents, sizing agents, opacifiers and the like. In many instances, more than one chemical additive is added to the product at some point in the manufacturing process.
Further, some chemical additives simply do not bond well with the cellulosic materials, such as cellulosic fibers. For instance, problems have been experienced in the past in bonding humectants and softeners to paper webs. Humectants have utility in tissue products for improving tactile feel through plasticization of fibers by increasing the humectancy of the finished web. Examples of humectants include, for instance, polyhydric alcohols such as propylene glycol, and polyethers such as poly (ethylene glycol), poly (propylene glycol) and their corresponding co-polymers. Since these materials are generally non-ionic and have no charge, there are poorly retained by cellulose fibers and typically cannot be added in the wet end of a paper making process.
Similarly, softeners such as polysiloxanes also fail to carry a charge necessary to form a strong ionic bond with cellulosic materials. Recently, in order to improve polysiloxane retention on paper webs and in order to improve the properties of the polysiloxanes, the polysiloxanes have been amino-functionalized. Still, retention of amino-modified polysiloxanes on cellulose fibers contained in an aqueous slurry, in some applications, is no better than about seventy percent (70%). Retention can also be dependent on aging (storage time) of the amino-polysiloxane as well.
A cellulose papermaking fiber primarily contains two types of functional groups, hydroxyl and carboxyl. At a typical papermaking pH of about 4 to about 9 a portion of the carboxyl groups are ionized causing the cellulose papermaking fibers to possess a net anionic charge. These anionic sites on the cellulose fibers serve as the source of attachment for wet end chemical additives. The amount of carboxyl groups on the cellulose fibers is limited and depends on the nature of the pulp. In general bleached kraft pulps contain about 2 to about 4 milli equivalents of carboxyl per 100 grams of pulp while mechanical pulps may contain upwards of about 30 to about 40 milli equivalents of carboxyl groups per 100 grams of pulp.
Most wet end chemical additives used in papermaking rely on electrostatic interaction for retention of the additive to the papermaking fibers. In general, the chemical additives will possess a positive charge somewhere on the molecule. The positive charge is attracted to the negative charge on the cellulose fibers and an electrostatic interaction retains the chemical additives on the cellulose fibers. Where anionic chemical additives are used, a cationic promoter will be used to bridge the anionic chemical additive and the anionic sites on the cellulose fibers. The limited number of carboxyl groups on the cellulose fiber limits the amount of chemical additives that can be retained on the cellulose fibers in addition to problems experienced with chemical additives that have weak anionic properties to begin with. Also, where more than one chemical additive is used in the wet end, competition between the two chemical additives for the limited number of bonding sites on the cellulose fibers can result in inconsistent retention leading to variable product performance.
When added in the wet end, non-ionic chemical additives as described above show poor retention to the cellulose papermaking fibers. An option to circumvent this issue is to covalently bond the molecule to the cellulose fibers in some way. A problem with covalent bonding to cellulose lies in the type of groups on the cellulose fibers that are available for reaction. The two chemically active groups on the cellulose fibers are hydroxyls and carboxyls. The carboxyl groups are generally too few in number and too low in reactivity to be useful. Also, any reaction at the carboxyl group will reduce the number of available ionic bonding sites on the cellulose fibers hence limiting the ability to retain any charged wet end chemical additives that may need to be used. The hydroxyl groups, while plentiful, are problematic in that anything that can react with a hydroxyl group can also react with water. In a typical papermaking process, on a molar basis, the amount of the hydroxyl groups on water available for reaction is magnitudes of order larger than the amount of the hydroxyl groups of the cellulose fibers available for reaction. Simple kinetics will therefore dictate a preference for reaction with water hydroxyl groups over the cellulose fiber hydroxyl groups. This problem can be overcome as exemplified with the sizing agents ASA (alkyl succinic anhydride) and AKD (alkyl ketene dimer). However, complicated and expensive emulsification must be performed in order to allow addition of these chemical additives to the wet-end of the process. The costs become prohibitively high for use in tissue. Additionally, such materials generally react with the hydroxyl groups of the cellulose fibers only after the forming process and removal of a majority of the water. Therefore the emulsions are cationic and the chemical additive is retained in the non-reacted state due to the attraction of the cationic emulsion for the anionic sites of the cellulose fibers. Hence, even in this case the amount of anionic sites on the cellulose fibers available for bonding with other charged wet end chemical additives is reduced.
Therefore, there is a need for a means of retaining higher and more consistent levels of paper modifying chemical additives on the paper web via wet end addition or topical application. Furthermore, there is a need for retaining more than one chemical functionality to a paper web that mitigates the limitations created by the limited number of bonding sites. There is also a need in the art for a method for bonding generally non-ionic additives to cellulosic materials and, in particular, a need exists for a method for bonding humectants and softeners onto cellulosic materials.